Upward facing Lithium Flash Evaporator for NSTX_U*

AbstractAbstract.

SOFE June 10th – Jun 14th, 2013 • San Francisco, CA

*Work supported by U.S.DOE Contract No. DE-AC02-09CH11466

NSTX plasma performance has been significantly enhanced by lithium conditioning.1

To date, the lower divertor and passive plates have been conditioned by downward facing lithium evaporators (LITER) as appropriate for lower null plasmas. The higher power operation expected from NSTX _U requires double null plasma operation in order to distribute the heat flux between the upper and lower divertors making it desirable to coat the upper divertor region with Li as well. An upward aiming LITER (U-LITER) is presently under development and will be inserted into NSTX using a horizontal probe drive located in a 6” upper port of one of the large midplane port covers. The operation of the evaporator is designed to minimize coatings on diagnostic windows. In the retracted position the evaporator will be loaded with up to 300 mg of Li granules utilizing one of the calibrated NSTX Li powder droppers2. The evaporator will then be inserted into the vessel in a location within the shadow of the RF limiters and will remain in the vessel during the discharge. About ~10 seconds before a discharge it will be rapidly heated and the lithium completely evaporated, thus avoiding the complication of a shutter to prevent evaporation during the shot when the diagnostic shutters are open. The minimal time interval between the evaporation and the start of the discharge will avoid the passivation of the lithium by residual gases and enable the study of the conditioning effects of un-oxidized lithiated surfaces. The evaporator will be withdrawn, reloaded with Li granules and reinserted during the inter-shot interval.

Two methods are being investigated to accomplish the rapid (few second) heating of the lithium. A resistive method relies on passing a large current through a Li filled tungsten crucible. A second method requires using a 3 kW e-beam gun to heat the Li. In this paper the evaporator systems will be described and the pros and cons of each heating method will be discussed.

A. L. Roquemore, D. Andruczyk, R. Majeski, D.K. Mansfield, C.H. Skinner, Douglas Rodgers** Princeton Plasma Physics Laboratory, Princeton, NJ 08543-0451

Goals for the flash evaporator:

• Electron beam

– It will require about 6.18 kJ to heat this from room temperature (20 oC) to boiling temperature

– With a 3kV electron beam it will take about 2.7 s to achieve this.

• A rough calculation of evaporation rate, gives 387 gm-2s-1

• Intention is to evaporate approximately 300 mg of lithium (an aspirins worth)

– So to evaporate this it will take about 0.7 s

• Total time to melt and evaporate 3.5 s

• Resistive Heating

– Tungsten boat mW = 15 g

– Current I = 500 A

– Voltage V = 3 V

– Power P = 1500 W

– Requires about 2700 J to go from 20 oC to 1342 oC

• It will take about 1.8 s to heat the tungsten boat to the boiling temperature of lithium.

– Assuming that Li also melts to this temperature in roughly the same time and the evaporation rate is the same as above

• 2.5 – 3 s to flash evaporate

Beam Footprint as function of Helmholtz coil radius

Helmholtz configuration

Lithium Evaporation rate for different beam energy

Vary Accel Voltage, fixed current at .75

Amps

Vary Total Power densityFrom 10, 80,150, 225

300 MW/m2

• Cannot use permanent magnets inside NSTX-U• Specifications for the magnetic field

– Ee = 3-5 kV electron beam– Diameter d = 30 mm (1”)

• Upper limit– For a beam to be bent with a 30mm diameter will require a magnetic field

of:– B = 90-150 gauss

• Magnetic field will be provided by electromagnets– Set of Helmholtz Coils– 57.15 mm (2.25”) average diameter– 10 mm wide– Assume ~2 A to go through the coils. How many turns (N) are needed.

• Parameters– Current, I = 2 A– Resistance, R = 2.4 A– Voltage, V = 4.8 V– Power, P = ~ 10 W

• N = 216 turns required for each coil.

What is the over all temperature increase, ΔT, for a 10s pulse of the U-LITER (These are maximum parameters).

• Calculations show that, a rise of ΔT = 2 oC rise in the temperature

• A full run day:

− 40 shots

− 10 s to run the Helmholtz coils

− Assume radiative cooling between shots

− Will eventually have a 80 oC temperature rise

• Ttot = 100 oC

− Acceptable for vacuum compatible polyamide coated wire.

Comparison between Resistive and e-beam heaters

Comparison between Present LITER and U-LITER

Larger footprint reduces power density and

eliminates need for beam sweeping

** E-beam calculations performed as partial fulfillment of Bachelor of Science degree at Drexal University

Crucible orientation to coat upper divertor.

Ratio of crucible depth to opening is 2:1 and allows a 40 degree spray.